We dare you to not look at your hand after reading this: Guys with shorter index fingers than ring fingers tend to have bigger penises than guys with the opposite digit scenario, finds a new study in the Asian Journal of Andrology.

How did researchers get this information? Very carefully. While men who came in to a Korean hospital for urological surgery were anesthetized, scientists measured consenting patients’ penises. (“Thanks for coming in for penis surgery! Mind if we measure it while you’re knocked out?”)

Fun fact: Since shorter members stretch more than longer ones, the stretched measurement—used by the researchers—is more telling about the size of an erect penis. Other researchers measured the patients’ index and ring fingers.

The connection: The testosterone men get exposed to while they’re still fetuses controls both penis and finger length, the researchers explain.

The early testosterone exposure reflected in your fingers (and elsewhere) affects you in many other ways, explains Denise Brooks McQuade, a scientist at Skidmore College not affiliated with the study.

Guys with shorter index finger than ring fingers—known as a low digit ratio—tend to have a higher risk of prostate cancer, be better at sports and financial trading, and have higher sperm counts.

We know by now you’ve 1) Looked at your hand and 2) Adjusted your hand’s position so that your index finger looks shorter.

Want to figure out your digit ratio for real? Watch this video from Men’s Health to learn how to accurately measure it. (You can quit feeling self-conscious now: The average size of an erect penis is a not-at-all-intimidating-to-think-about 5.5 inches.) Then take our Prostate Cancer Quiz to get a better idea of your risk.